1. Technical Field
The present invention generally relates to methods of designing tread patterns for tire noise. More particularly, the invention relates to a method for designing tread pattern tire noise pitch sequences by pre-selecting variations in lug stiffness characteristics. The invention specifically relates to a method for designing tire noise pitch sequences to achieve preferred characteristics of variations in tread lug stiffness based on the distribution of the lugs in the tire noise pitch sequence.
2. Background Information
One aspect of tire design involves minimizing undesirable tire noise. Tire noise is generated when the lugs of the tread pattern contact the road surface. An unvarying tread pattern, or mono pitch tread pattern, creates an undesirable tonal, or mono pitch sound. Tire designers vary the tread pattern to avoid mono pitch sounds. The tread patterns are typically varied by altering the size of the tread pitches around the circumference of the tire. Varying the sizes of the tread pitches tends to reduce mono pitch tire noise by broadening the frequency domain of the noise spectrum but undesirable noise in the time domain can still be created.
Tread patterns are currently analyzed by comparing the tire noise generated by different variations in the tread pitches. Known analysis techniques allow the tire designer to select a pitch pattern for the tread design that generates acceptable tire noise. One such technique uses the Fourier spectrum of the pitch sequence to identify objectionable pitch sequences. Another technique disclosed in U.S. Pat. No. 6,112,167 analyzes portions of the pitch sequence around the circumference of the tire. Although these techniques have been effective, tire designers have found that known techniques can result in tire noise pitch sequence designs that pass initial screening while still having undesirable tire noise. Tire molds having such sequences must be rebuilt or modified to reduce the undesirable noise. One cause of undesirable noise is tire noise attributed to the variations in the lug stiffness around the circumference of the tire.
When the size of the tread pitches is varied, the size of the lugs of the tread pattern is varied. The lugs thus have different stiffnesses and produce different sound amplitudes when they contact the road surface. These differences create sound amplitude variation that can make an otherwise desirable pitch sequence undesirable. In the past, this undesirable tire noise was not analyzed and tires would be produced before the undesirable noise was recognized. If the customer objected to the noise, the tire manufacturer would have to scrap the expensive tire mold or would have to modify the mold. In response to this issue, the art thus desired a secondary screening method that allows the tire designer to compare a group of otherwise desirable pitch sequences. This screening technique is disclosed in pending patent application US 2003/0040886 A1 dated Feb. 27, 2003, which describes a method for comparing tread designs based on tire noise generated by tire tread lug stiffness variation. The art thus desires a method to develop tread designs with pre-selected lug stiffness variation characteristics. Such a tread pattern design method would define tire noise pitch sequences for optimized lug stiffness variations and tire noise level characteristics. FIGS. 1A-C and 2A-C demonstrate the inherent difficulties in defining tire noise pitch sequences with optimized lug stiffness variations and tire noise level characteristics. FIGS. 1A-C provide for a 60-pitch noise sequence with good level characteristics, but poor lug stiffness (further referred to as modulation) characteristics. Good level characteristics consist of a smooth spectrum about the first tread passage harmonic range that is centered at 60 harmonics for 60 pitches. Notice that for the sequence depicted in FIG. 1C, there is relatively high modulation levels at the first and second modulation order. These high levels would translate to a tire tread pattern with a strong once and twice per revolution noise variation. A tire pattern with good modulation characteristics would have low modulation levels. FIGS. 2A-C demonstrate a tire noise pitch sequence with good modulation characteristics. An analysis of the harmonic content of this sequence provides for a narrow, tonal band of energy about the 60th, 120th, 180th harmonic and subsequent multiples of 60 harmonics with high, undesirable, level characteristics.